In a voice communication using packets such as VoIP (Voice over IP), a encoding scheme having frame loss tolerance when encoding voice data is desired. This is because in a packet communication represented by Internet communication, packets are sometimes lost in a transmission path due to congestion or the like.
As one of methods for increasing frame loss tolerance, there is an approach which makes influences of frame loss as small as possible by performing decoding processing from other parts even when some part of transmission information is lost (for example, see Patent Document 1). Patent Document 1 discloses a method of transmitting core layer encoded information and enhanced layer encoded information packed in separate packets using scalable encoding. Also, one of packet communication applications is a multicast communication (one-to-many communication) using a network on which thick channels (broadband channels) and thin channels (channels of low transmission rates) coexist. Even when communications are carried out among many spots on such heterogeneous networks, if encoded information is hierarchically structured in accordance with the respective networks, there is no necessity for sending encoded information which differs for every network, so that scalable encoding is effective.
As an example of a band scalable encoding technology which has scalability in the signal bandwidth, that is, in the frequency axis direction based on a CELP scheme which enables high efficiency encoding of a voice signal, there is a technology disclosed in Patent Document 2. Patent Document 2 shows an example of a CELP scheme which expresses spectral envelope information of a voice signal using LSP (line spectrum pair) parameters. Here, a band scalable LSP encoding method is realized by converting quantized LSP parameters (narrowband encoding LSP) obtained at a encoding section (core layer) for narrowband voice to LSP parameters for wideband voice encoding using following (Expression 1) and using the converted LSP parameters at a encoding section (enhanced layer) for wideband voice.fw(i)=0.5×fn(i)[i=0, . . . , Pn−1]=0.0[i=Pn, . . . , Pw−1]  (Expression 1)where fw(i) denotes an ith-order LSP parameter in a wideband signal, fn(i) denotes an ith-order LSP parameter in a narrowband signal, Pn denotes an LSP analysis order of the narrowband signal and Pw denotes an LSP analysis order of the wideband signal, respectively.
However, since Patent Document 2 explains a case where the sampling frequency is 8 kHz for a narrowband signal, the sampling frequency is 16 kHz for a wideband signal and the wideband LSP analysis order is twice the narrowband LSP analysis order as an example, the conversion from narrowband LSP to wideband LSP can be performed using a simple expression as shown in (Expression 1). However, since the position where a Pnth-order LSP parameter on the low-order side of wideband LSP exists is determined for the whole wideband signal including a (Pw−Pn)th order on the high-order side, it does not always correspond to the Pnth-order LSP parameter of narrowband LSP. For this reason, the conversion shown by (Expression 1) is not able to obtain high conversion efficiency (which may also be referred to as “prediction accuracy” if wideband LSP is predicted from narrowband LSP), and a wideband LSP coder designed based on (Expression 1) leaves room for improving encoding performance.
For example, Non-Patent Document 1 discloses a method of determining optimum conversion coefficient β(i) per order using an algorithm of optimizing the conversion coefficient as shown in following (Expression 2) instead of setting the conversion coefficient by which the ith-order narrowband LSP parameter in (Expression 1) is multiplied to 0.5.fw—n(i)=α(i)×L(i)+β(i)×fn—n(i)  (Expression 2)where fw_n(i) is the ith-order quantized wideband LSP parameter in an nth frame, α (i)×L(i) is an ith-order element of a vector obtained by quantizing a predicted error signal element (α (i) is an ith-order weighting factor), L(i) is an LSP predictive residual vector, β (i) is a weighting factor for prediction wideband LSP and fn_n(i) is a narrowband LSP parameter in the nth frame. By such optimization of a set of conversion coefficients, although it is an LSP coder having the same configuration as Patent Document 2, higher encoding performance is realized.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-241799    Patent Document 2: Japanese Patent Application Laid-Open No. HEI 11-30997    Non-Patent Document 1: K. Koishida et al, “Enhancing MPEG-4 CELP by jointly optimized inter/intra-frame LSP predictors,” IEEE Speech Encoding Workshop 2000, Proceeding, pp. 90-92, 2000